A battery pack including a chargeable-dischargeable battery string and a battery management system (BMS) that monitors and controls charging and discharging of the battery string is mounted in, for example, hybrid cars, electric vehicles, and so on. Such a battery pack mounted in a vehicle is charged with power generated by a power generator (regeneration device) of the vehicle.
To increase the capacity of the whole power supply system including a battery pack, it is the case that a plurality of battery packs are provided within the power supply system and a relay provided in series to the assembled battery of each battery pack is turned on to thereby connect the plurality of battery packs in parallel. In this case, if the total voltage of the battery string differs between the battery packs by a potential difference ΔV, a circulating current is generated between the battery packs in accordance with the potential difference ΔV. When the magnitude of the circulating current exceeds the maximum current that the battery packs can withstand, a relay failure may occur. PTL 1 discloses a technique for allowing the battery packs to be connected with each other only in a case where the potential difference ΔV is equal to or smaller than a predetermined value.
To shut down the power supply system having a plurality of battery packs connected in parallel, a device of interest needs to be stopped first, and a relay provided in each battery pack needs to be turned off last. At the time point when the current between the device and the power supply system becomes zero or substantially no current flows between the device and the power supply system due to, for example, the stop of the device, if the total voltage of the battery string differs between the battery packs, the circulating current circulating between the battery packs flows in accordance with the difference.
When the relay within each battery pack is opened in a state where a large circulating current is flowing, the difference in the total voltage between the battery packs is maintained, which may be a cause of the circulating current described above generated when the power supply system is started up next time. PTL 2 discloses a technique in which, to equalize the voltage between the battery packs at the time when the device is shut down, the relay within each battery pack is left turned on for a while after the power supply system is electrically disconnected from the device to make the circulating current flow, and the relay within each battery pack is turned off after an elapse of a predetermined period.